1. Field of the Invention
The present invention relates to a magnetic recording medium capable of storing a large amount of information and also to a magnetic storage equipped therewith. More particularly, the present invention relates to a magnetic recording medium suitable for high-density magnetic recording and also to a magnetic storage equipped therewith.
2. Description of Related Arts
The advancing information society processes an ever-increasing amount of information, and there is a strong demand for a high-capacity magnetic storage. Energetic efforts are being made to meet this demand by both improving the sensitivity of the magnetic head and reducing the noise level of the magnetic recording medium. As for the magnetic recording medium, there has appeared a new one which replaces the conventional magnetic film of Co—Cr—Pt, Co—Cr—Ta, or Co—Cr—Pt—Ta alloy. For example, Japanese Patent Laid-open No. 221418/1992 discloses a magnetic recording medium for high density recording which is characterized by a cobalt-alloy magnetic layer containing at least platinum and boron for increase in coercive force. Also, Japanese Patent Laid-open No. 293227/1997 discloses a magnetic recording medium which has a Cr—Mo underlayer and a Co—Cr—Pt—B magnetic layer in combination. These inventions are intended to reduce the noise level of the recording medium through improvement in the material of the underlayer or the material of the magnetic layer. On the other hand, an approach to noise reduction is being made through reduction of grain size in the magnetic layer. The disadvantage of this approach is that recorded bits are thermally unstable. One possible way to avoid this disadvantage is to enhance the magnetic anisotropy of the magnetic film. However, its upper limit is determined by the writing magnetic field of the recording head.
For improvement of the thermal stability of recorded bits and reduction in media noise, Japanese Patent Laid-open Nos. 56924/2001 and 148110/2001 disclose a magnetic recording medium having on a substrate a recording layer composed of at least two magnetic layers which are anti-ferromagnetically coupled through a non-magnetic coupling layer. (This recording medium will be referred to as “anti-ferromagnetically coupled medium” hereinafter.) The anti-ferromagnetic coupling is realized when the non-magnetic coupling layer is a non-magnetic film of Ru or the like which has a thickness of 0.4 to 1.0 nm. The anti-ferromagnetically coupled medium offers the advantage of keeping the residual magnetic flux density low despite its large thickness. This is because the two magnetic layers holding the non-magnetic coupling layer between them are magnetized in mutually anti-parallel directions in the absence of an applied magnetic field. The result is high-density recording as well as good thermal stability.
The anti-ferromagnetically coupled medium is expected to have greatly improved thermal stability and recording resolution as mentioned above; however, it is not necessarily promising as far as improvement in media noise is concerned. For the recording medium with a reduced residual magnetic flux density, improvement in its S/N ratio is possible by preventing the read output from decreasing due to thermal decay but great reduction in media noise is not achieved yet.